Which Deworming Program is Right for Your Horse

Rotational Deworming, Strategic Dosing,
Continuous Treatment or Selective Therapy,
Which is the Best?

So Many Parasites, So Little Time

There are over 150 species of internal parasites or worms that can infect your horse. The most common equine parasites to watch for are small strongyles (redworms), ascarids (roundworms), and tapeworms (flatworms), and large strongyles (bloodworms).  

Following is a table that lists each of these parasites, their preferred location inside your horse, and the type of symptoms they cause.

Parasite Location Symptoms

  (Small Strongyles)

Mature in large intestine. Encysted larvae burrow into intestinal walls. Causing damage to digestive system

Malnutrition, weight loss, and diarrhea


  Roundworms (Ascarids)


Mature in small intestine. Migrate through intestinal walls to liver, then to lungs causing damage to both respiratory and digestive system


Coughing, malnutrition, fever, colic, slow growth in foals


  Flatworms (Tapeworms)

Mature in large intestine. Larvae migrate throughout the organs via the blood vessels, weakened abdominal artery walls and leaving behind ulcerated nodules.

Weight loss, anemia, ulceration, and colic


Large Strongyles)


Mature in large intestine. Larvae migrate throughout the organs via the blood vessels, weakened abdominal artery walls and leaving behind ulcerated nodules.


Weakness, weight loss, diarrhea, anemia, and colic


These parasites, left undetected, can wreak havoc on your horse’s internal organs, leading to an ulcerated digestive system, recurrent colic, irritated or damaged lungs, and blood vessel damage. Therefore, it is vital to develop a parasite control program specifically designed for your horse that targets and controls these parasites and worms.

Control Not Eliminate

The goal of any sustainable equine parasite control program, also known as worming or deworming programs, is not to eliminate all parasites but rather to control them.

There are three essential elements to consider when creating a sustainable parasite control program for your horse. Each of these elements plays a critical role in identifying, eliminating, and managing your horse’s internal parasite and worm burden:

  • Testing involves diagnosing the extent of your horse’s internal parasitic infection. You can accomplish this by performing regularly scheduled fecal egg count testing. Fecal egg count test results reveal the type and number of parasite eggs present in your horse’s feces. This information allows you to determine if your horse requires deworming. If a deworming treatment is needed, a post-treatment fecal egg count reduction test can also be conducted to gauge the effectiveness of your chosen dewormer.
  • Treatment consists of the selection, dosage, and application of the appropriate equine dewormer. A dewormer or wormer is an over-the-counter chemical that you administer to control internal parasites or worms. It is important to note that different deworming products target specifics worms and parasites. So be sure to read the wormer product label and literature to makes sure you select a dewormer that targets the appropriate parasites.
  • Environment. What goes in your horse will eventually come out of your horse. Keeping your horse’s stall, pasture and dry lot, manure free year-round is critical for guarding against parasitic infection as well as preventing reinfection. Removing feces from stalls and pastures on a regular basis can nearly eliminate the need for deworming treatments.

Deworming Programs - The Beginning

Accompanying the 1960’s introduction of broad‐spectrum equine anthelmintics (dewormers that are effective against multiple types of horse parasites), was the development of treatment programs supposedly designed to totally eliminate your horse’s parasites. However, these programs did not consider the type or number of parasites that may be infecting your horse nor, did they provide any feedback necessary to measure the effectiveness of any treatments.

Research over the past few decades has demonstrated that due to the parasites being a permanent resident in your horse's environment, it is virtually impossible to maintain a parasite‐free horse property, and continued attempts to totally eliminate parasites and worms by repeated use of a chemical dewormer only accelerates parasitic resistance on a property.

Different Strokes for Different Folks

Following are four deworming approaches that have been applied historically in an attempt to control horse parasites:


*** WARNING ***
Scientific Jargon Below

A horse's Egg Reappearance Period (ERP) is defined as the time interval between the last effective anthelmintic (dewormer) treatment and the resumption of significant parasite egg (i.e., strongyle) shedding.

1. Rotational Deworming

Rotational deworming programs, which are also referred to as interval‐dosing or blind deworming programs, unfortunately, became the universal standard in managed horse populations worldwide due to how simple and easy they are to implement.

Rotational programs use a one‐size‐fits‐all, cook‐book deworming recipe which treat all horses the same regardless of complicating factors such as the type and number of parasites, age of horses, stocking densities, or seasonal variation.

A rotational program's objective is to inhibit the spread of parasites by treating a horse with a different deworming product every couple of months (i.e., rotating) based on that product’s Egg Reappearance Period (ERP) - See the side bar for further information about ERP.

Table 1 below shows the ERPs for four dewormers available in the United States. The table's middle column demonstrates the ERPs for these four dewormers have shrunk since their introduction. The decreasing ERP timeframes are assumed to be an indication that parasite resistance is developing to these dewormers.

Dewormer Usual ERP when drug is effective ERP when drug was first introduced


  4 to 5 weeks

  6 weeks


  4 to 5 weeks

  5 to 6 weeks


  6 to 8 weeks

  9 to 13 weeks


  10 to 12 weeks

  16 to 22 weeks

 Table 1- Small strongyle egg reappearance periods (ERP) equine dewormers - AAEP Parasite Control Guidelines

A major drawback to rotational deworming programs is that instead of testing, these programs recommend rotating deworming products with each treatment at fixed intervals (i.e., every two to three months) all year long. The intent of these programs was to provide proactive coverage against the major types of internal parasites such as blood/redworms, roundworms, and tapeworms that may infect your horse. However, without testing, research now shows that this frequency of treatment is unnecessary, and repeated exposure to excessive toxic chemicals is both wasteful and potentially harmful to your horse.

Additionally, rotating dewormers was supposed to slow parasitic resistance to the products being used. However, the opposite is happening, due to the frequency of use, and some worms have become resistant to certain dewormers.

Because rotational deworming does not include feedback (i.e., testing), most equine parasitologists (these are the folks that study horse parasites) now believe rotational deworming programs are the major factor responsible for the acceleration of parasitic dewormer resistance on horse properties, and therefore should not be considered for long term programs.

2. Strategic Dosing

A strategic dosing program is similar to rotational deworming programs but takes seasonal differences into consideration. Meaning this type of program takes into account when the parasites are most likely to reproduce and therefore shed into a horse's surroundings. In strategic dosing programs, treatments are primarily administered during the active grazing season when parasites are reproducing due to favorable weather. However, these treatments are still performed without testing regardless of the horse's age, health, and surrounding.

Strategic dosing programs do lower the number of treatments, but like rotational deworming, all horses receive the same treatments at fixed times during the year.

3. Continuous, Daily Treatment

A continuous, daily treatment program involves the administration of a dewormer to all horses, either on a year-round daily basis or just daily throughout the grazing season. A continuous program's objective of a daily administration of a dewormer is to kill the ingested immature and recently emerged adult parasites before they invade and damage your horse's internal organs.

Though horses receiving daily treatments should maintain a low egg count while receiving the treatment, recent investigations have shown that even with daily dosing, parasite egg counts are not reduced to zero, and seasonal or gradual increases are still observed. Repeated use of the same daily deworming will also accelerate parasitic resistance to that product.

4. Selective Therapy

As compared to the three other deworming programs, selective therapy reduces the number of treatments and thereby slows parasitic resistance on a horse property by considering the type and number of parasites infecting a horse.

A recent National Animal Health Monitoring Systems for equines has shown that 80% of the parasites on a property come from up to 30% of the horses (see Table 2).

Classification based on egg shedding Egg Count (per gram of feces) % of horse population


    0 - 200

  50% to 75%


    200 - 500

  5% to 15%



  10% to 30%

Table 2- Suggested guidelines for classifying horses into different levels of strongyle egg shedding and the expected percentage of the horse population belonging to each group (Kaplan and Nielsen, 2010).


Since higher shedders are responsible for the majority of parasite eggs shed in their environment, it makes sense to treat them more often than lower shedders.

Selective therapy abandons whole‐herd one-size-fits-all treatment approaches and designates only certain horses for deworming therapy. Selective therapy uses testing to determine a horse infection intensity or shedder category (see Table 2 for egg shedding classification limits.)

A selective therapy program also uses a concept called refugia, which leaves a population of a property's horses untreated (see sidebar for more on refugia).

All horses on a property are tested when following a selective therapy approach. This method also recognizes that each horse possesses a unique immunity to some parasites. Therefore, each horse’s treatment is based on their test results, and treatment is only administered to those horses with egg counts exceeding a predetermined cut‐off value (e.g., 200 EPG.)

Refugia: Leaving some parasites unexposed to a dewormer (essentially giving them refuge) which allows non-resistance parasites to breed with resistance parasites thereby slowing the resistance.

 Which Program is the Best

Table 3 below compares the equine parasite control programs described in this blog. The top row of the table lists the four deworming programs. The left-most column lists the deworming programs' characteristics. The remainder of the table indicates whether a feature is part of that program.

Characteristics Rotational Strategic Continuous Selective

Horses Receive Same Treatment





Prescribed Treatment Intervals

Yes - every 2 or 3 months

Yes - every 2 or 3 months

Yes - daily

No - depends on test results

Parasite Surveillance




Yes – fecal egg count tests





Yes - Shedding Categories (H/M/L)

Deworming Effectiveness





Seasonal Differences





Creates Refugia





Table 3- A characteristic comparison of four popular equine parasite control programs.


Controlling exposure to harmful parasites is a critical step in keeping your horse healthy. Part of keeping your horse healthy is implementing a proactive health care plan that includes a targeted, intelligent approach to parasitic management. Your equine parasitic management program's goal should focus on preventing or reducing parasite transmission, not eliminating the parasite.

Just like your horse is special and unique to you, so should your horse’s deworming program be unique. Since every horse is different, there is no one-size-fits-all deworming program. Instead, the AAEP Parasite Guidelines now recommend properly timed treatments based-on accurate fecal egg count testing results. Additionally, a comprehensive equine parasite program should include an acceptable level of parasite refugia while focusing on a goal of sustainable control rather than parasite elimination.

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